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<br /> <br />Wright Water Engineers, Inc., 818 Colorado Ave., Ste. 307, Glenwood Springs, CO 81601 <br />Tel. (970) 945-7755; Fax. (970) 945-9210, e-mail: jkelly@wrightwater.com <br /> <br /> MEMORANDUM <br />To: Jessica Wilczek, P.E. <br /> Mountain Coal Company <br /> Via Email <br />From: Wright Water Engineers, Inc. <br /> Jonathan Kelly, P.E. <br />Date: June 14, 2024 <br />Re: Hydrologic Consequences of Pillaring in the Sunset Mains <br />Wright Water Engineers, Inc. (WWE) has evaluated the hydrologic consequences of pillaring in the <br />Sunset Mains, north of longwall panel E14. WWE had previously analyzed the subsidence associated <br />with the longwall mining in the adjacent panels and found that the mining would not have adverse <br />effects on the hydrology of the overlying stream channels. Our findings were documented in Exhibits <br />55B and 60E of the mine permit. <br />Agapito Associates modeled the projected subsidence resulting from the pillaring in the mains. Their <br />results indicate that the maximum subsidence would be up to 2.5 feet, and this would be limited in <br />areal extent. WWE overlaid the Agapito modeling results to analyze the potential impact on the stream <br />channels in the vicinity of the pillaring activity. <br />The headwaters of Deer Creek are within the influence of the projected subsidence resulting from the <br />pillaring activity. The areas where maximum subsidence is projected are not overlain by stream <br />channels, and the maximum subsidence underlying the stream channels is limited to 1 to 1.5 feet. <br />WWE evaluated the potential for the projected subsidence to impact the stream channels within these <br />drainages and found that there should not be hydrologic impacts. Importantly, these streams have <br />previously been undermined by longwall mining that resulted in significantly larger subsidence <br />without experiencing any change in flow characteristics. The steepness of the channel grades and the <br />ability of the alluvial and colluvial materials to heal and/or seal following differential subsidence has <br />resulted in no observable impacts to these streams. The small amount of projected subsidence <br />resulting from the pillaring in the Sunset Mains is not expected to adversely affect the hydrology of <br />Deer Creek drainage. <br /> <br />G:\WWE\831-032\Sunset Pillaring\Sunset Pillaring Mains\Sunset Mains Pillaring.docx